In conventional practice, a protective film is attached to an entirety of a front surface of a wafer in order to protect devices formed on the surface of the wafer (see Japanese patent laid-open publication No. 2005-303158 and Japanese patent laid-open publication No. 2005-317570). A protective film may be attached to an entirety of a back surface (i.e., a surface opposite to the front surface on which devices are formed) in order to protect the back surface from a liquid chemical (e.g., a plating solution) and polishing debris. In a plating process, for example, a wafer, having a protective film attached to the back surface thereof, is immersed in a plating solution, and in this state plating of the wafer is performed.
However, the protective film can sometimes peel off the wafer during processing of the wafer. For example, since the plating solution is heated to a certain processing temperature, an adhesive power of an adhesive of the protective film is lowered, and as a result a peripheral portion of the protective film may peel off the wafer. Once the peripheral portion of the protective film has peeled off, the plating solution will intrude into a gap between the back surface of the wafer and the protective film. As a result, metal ions contained in the plating solution may adhere to the wafer back surface composed of silicon and may diffuse into the wafer, resulting in performance failure of devices.
These days, there is a demand for protecting not only front and back surfaces of a wafer but also a peripheral portion of the wafer. For example, in a process of forming trenches in a wafer by dry etching, a plasma is generated on the surface of the wafer in the presence of an etching gas, and trenches are formed at desired positions in the wafer using a resist as a mask. In this dry etching process, pillar structures of silicon, called “black silicon”, may be formed on a peripheral portion of the wafer where no resist exists. Such black silicon may fall off the peripheral portion of the wafer e.g., when the peripheral portion is gripped by a transport machine. The fallen black silicon may adhere to devices formed on the wafer, causing a defect, such as short circuit, in the devices.